The present invention relates to a resonator type surface acoustic wave filter used as, for example, a bandpass filter or other type of filter. More specifically, the present invention relates to a resonator type surface acoustic wave filter including a plurality of longitudinally coupled surface acoustic wave resonators which generates and uses a surface acoustic wave that has a Shear Horizontal (SH) wave as its main component, such as, a Bleustein-Gulyaev-Shimizu (BGS) wave.

Various conventional surface acoustic wave filters are known bandpass filters. For example, a transversely coupled surface acoustic wave filter using a Rayleigh wave as a surface acoustic wave, and transversely coupled and longitudinally coupled resonator filters using a Rayleigh wave are already known bandpass filters.

The transverse type surface acoustic wave filter is constructed such that a fixed distance separates a pair of interdigital transducers (IDTs) from each other on a surface acoustic wave substrate.

Further, in the transversely coupled resonator filter using a Rayleigh wave, a plurality of surface acoustic wave resonators include a plurality of IDTs that are provided on a surface acoustic wave substrate with the plurality of surface acoustic wave resonators coupled in the transverse direction. In the transversely coupled resonator filter, reflectors are located on both sides of an area occupied by the IDTs.

Recently, an edge reflection type surface acoustic wave filter using an SH type surface acoustic wave such as a BGS wave has been developed because the filter has the advantage of being compact.

In a transversely coupled surface acoustic wave filter using the Rayleigh wave, because two sets of IDTs are separated by a fixed distance along the direction of the surface-wave transmission on a surface acoustic wave substrate, the size of the filter cannot be reduced and is instead much larger. Also, the insertion loss is very large.

In a transversely coupled resonator filter using a Rayleigh wave, because the electromechanical coupling coefficient K is small, the filter does not have sufficient bandwidth, and although the loss can be reduced, the aperture of the electrode fingers of the IDT is about 10 λ and the coupling length is about 1.5 λ assuming that the wavelength of the surface acoustic wave is λ. Additionally, reflectors, as described above, are required. Accordingly, the size of the filter cannot be reduced and made compact.

Further, in an edge reflection type surface acoustic wave resonator using a SH type surface acoustic wave, a resonator with a wide bandwidth is obtained, which is due to the relatively large electromechanical coupling coefficient K, so that no reflectors are required. Further, the edge reflection type resonator is favorable because it can be made compact. However, up until now, when a SH type surface acoustic wave was used, the construction of a filter by coupling the plurality of surface acoustic wave resonators was considered to be impossible.

The pre-characterizing portion of claim 1 is based on DE 198 38 573.

To overcome the problems of the related art described above, preferred embodiments of the present invention provide a resonator type surface acoustic wave filter constructed so as to make use of a SH type surface acoustic wave, particularly a BGS wave, and which is miniaturized and compact size with very small insertion loss.

A longitudinally coupled resonator type surface acoustic wave filter according to the present invention includes a surface acoustic wave substrate, and first and second surface acoustic wave resonators provided on the surface acoustic wave substrate and utilizing a surface acoustic wave having an SH wave as a main component, and the first and second surface acoustic wave resonators are coupled to constitute a stage of the longitudinally coupled resonator filter, wherein the first and second surface acoustic wave resonators include first and second interdigital transducers arranged in the direction of transmission of the surface acoustic wave on the surface acoustic wave substrate, respectively, each of the interdigital transducers defined by first and second comb electrodes having a first bus bar and a second bus bar, respectively, and such that the bus bars are extended in the direction of the transmission of the surface acoustic wave, and each of the bus bars are connected to a first end of a plurality of electrode fingers such that a second end of the electrode fingers of the first bus bar is extended towards the second bus bar and a second end of the electrode fingers of the second bus bar are extended towards the first bus bar such that the electrode fingers of the first and second bus bars interdigitate with each other, characterized in that the surface acoustic wave substrate has a relative dielectric constant E such that E = ε^s₁₁/ε₀, and where E is less than or equal to 3000, and an aperture y is defined as an amount of overlap between the electrode fingers of the first bus bar and the second bus bar normalized by a wavelength λ of the surface acoustic wave, and wherein y satisfies the formula: $$\mathrm{5.52}\mathrm{+}\mathrm{66.62}\mathrm{\times }\exp \left(\mathrm{-}\mathrm{E}\mathrm{/}\mathrm{110}\right)\mathrm{\ge }\mathrm{y}\mathrm{\ge }\mathrm{0.80}\mathrm{+}\mathrm{3.48}\mathrm{\times }\exp \left(\mathrm{-}\mathrm{E}\mathrm{/}\mathrm{404}\right)$$

Also, when K is the electromechanical coupling coefficient of the surface acoustic wave substrate, preferably F = K² · ε^s₁₁/ε₀ is less than or equal to 250. and the aperture y preferably satisfies the following formula: $$\mathrm{7.96}\mathrm{+}\mathrm{44.14}\mathrm{\times }\exp \left(\mathrm{-}\mathrm{F}\mathrm{/}\mathrm{38.3}\right)\mathrm{\ge }\mathrm{y}\mathrm{\ge }\mathrm{0.40}\mathrm{+}\mathrm{4.35}\mathrm{\times }\exp \left(\mathrm{-}\mathrm{F}\mathrm{/}\mathrm{80}\right)$$

Further, the filter is preferably constructed so that the number of pairs N of electrode fingers of the first and second interdigital transducers satisfies the following formula: $$\mathrm{114.4}\mathrm{+}\mathrm{0.02}\mathrm{\times }\mathrm{E}\mathrm{-}\mathrm{9.283}\mathrm{\times }{\mathrm{10}}^{\mathrm{-}\mathrm{5}}\mathrm{\times }{\mathrm{E}}^{\mathrm{2}}\mathrm{\ge }\mathrm{N}\mathrm{\ge }\mathrm{2.0}$$

Alternatively, if the filter is constructed so that F = K². ε^s₁₁/ε₀ is less than or equal to 250, then the number of pairs of electrode fingers N of the first and second interdigital transducers preferably satisfies the following formula: $$\mathrm{-}\mathrm{171}\mathrm{+}\mathrm{292}\mathrm{\times }\exp \left(\mathrm{-}\mathrm{F}\mathrm{/}\mathrm{672}\right)\mathrm{\ge }\mathrm{N}\mathrm{\ge }\mathrm{2.0}$$

Additionally, the distance X between the adjacent electrode fingers of the first and second transducers preferably satisfies the following formula: $$\mathrm{-}\mathrm{5.423}\mathrm{+}\mathrm{5.994}\mathrm{\times }\exp \left(\mathrm{-}\mathrm{E}\mathrm{/}\mathrm{22894}\right)\mathrm{\ge }\mathrm{X}\mathrm{\ge }\mathrm{0.255}\mathrm{-}\mathrm{0.19}\mathrm{\times }\exp \left(\mathrm{-}\mathrm{E}\mathrm{/}\mathrm{446}\right)$$

Also, if the filter is constructed so that F = K² . ε^s₁₁/ε₀ is less than or equal to 250, then the distance X preferably satisfies the following formula: $$\mathrm{0.364}\mathrm{+}\mathrm{0.198}\mathrm{\times }\exp \left(\mathrm{-}\mathrm{F}\mathrm{/}\mathrm{67.5}\right)\mathrm{\ge }\mathrm{X}\mathrm{\ge }\mathrm{0.241}\mathrm{-}\mathrm{0.169}\mathrm{\times }\exp \left(\mathrm{-}\mathrm{F}\mathrm{/}\mathrm{58.1}\right)$$

Further, a longitudinally coupled resonator type surface acoustic wave filter is preferably constructed as an edge reflection type surface acoustic wave filter that makes use of reflection at a pair of opposing end surfaces of a surface acoustic wave substrate.

Further, the resonator type surface acoustic wave filter of preferred embodiments may include one stage or a plurality of stages.

In the longitudinally coupled resonator filter, first and second surface acoustic wave resonators that makes use of a BGS wave is constructed. And as ε^s₁₁/ε₀ is less than or equal to 3000 and the aperture Y is made so as to be within a certain range, excellent filter characteristics can be obtained.

Particularly, when K² . ε^s₁₁/ε₀ is less than or equal to 250 and the aperture Y is within a certain range, the number of pairs of electrode fingers of the first and second IDTs are within the desired range, and the distance X between adjacent electrode fingers of the first and second IDTs are a desired distance, greatly improved filter characteristics are achieved.

Because the filter is a resonator type surface acoustic wave filter of edge reflection type, reflectors are not required, and accordingly, it is possible to achieve a small-sized surface acoustic wave filter. Therefore, preferred embodiments of the present invention provide a surface acoustic wave filter having excellent filter characteristics while being compact.

In another preferred embodiment of the present invention, a communication apparatus having a duplexer includes the above described longitudinally coupled surface acoustic wave resonator filter.

Other features, elements and advantages of the present invention will be described in detail below with reference to preferred embodiments of the present invention and the attached drawings.

The present invention will become more fully understood from the detailed description given herein below and the accompanying drawings which are given by way of illustration only, and thus do not limit the present invention and wherein:

• Fig.1 is a plan view of a longitudinally coupled resonator type surface acoustic wave filter according to a first preferred embodiment of the present invention;
• Fig. 2 shows the relationship between the relative dielectric constant ε^s₁₁/ε₀ and the aperture length of the electrode fingers in a longitudinally coupled resonator type surface acoustic wave filter;
• Fig. 3 shows the relationship between K²·ε^s₁₁/ε₀ and the aperture length of electrode fingers in a longitudinally coupled resonator type surface acoustic wave filter;
• Fig. 4 shows the relationship between the relative dielectric constant ε^s₁₁/ε₀ and the number of pairs of the electrode fingers in the IDT of a longitudinally coupled resonator type surface acoustic wave filter;
• Fig. 5 shows the relationship between K²·ε^s₁₁/ε₀ and the number of pairs of electrode fingers in the IDT of a longitudinally coupled resonator type surface acoustic wave filter;
• Fig. 6 shows the relationship between the relative dielectric constant ε^s₁₁/ε₀ and the distance between the electrode fingers in a longitudinally coupled resonator type surface acoustic wave filter;
• Fig. 7 shows relationship between K²·ε^s₁₁/ε₀ and the distance between the electrode fingers in a longitudinally coupled resonator type surface acoustic wave filter; and
• Fig. 8 is a block diagram of a duplexer and a communication apparatus according to another preferred embodiment of the present invention.

Fig. 1 is a plan view showing a longitudinally coupled resonator type surface acoustic wave filter making use of a BGS wave as a first preferred embodiment of the present invention.

A resonator type surface acoustic wave filter 21 is constructed using a substantially rectangular surface acoustic wave substrate 22. The surface acoustic wave substrate 22 can be constructed using a piezoelectric substrate made up of piezoelectric material or by laminating thin films, such as ZnO thin films, on the substrate. In making the piezoelectric substrate, a piezoelectric single crystal such as LiTaO₃, LiNbO₃, or quartz, or piezoelectric ceramics such as lead titanate and zirconate type ceramics may be used. Further, when the surface acoustic wave substrate 22 is constructed by providing a thin film on the substrate, a thin film made of piezoelectric material such as ZnO, Ta₂O₃, or other similar materials is laminated on an appropriate insulating substrate such as alumina, sapphire, Si, or other similar materials. Alternatively, a piezoelectric substrate such as LiNbO₃, quartz, or other similar materials, or a thin film such as Au or other similar materials laminated on a piezoelectric substrate can be used. Note that when a surface acoustic wave substrate is constructed by laminating a piezoelectric thin film on the substrate, the IDTs are provided on the upper or lower surface of the piezoelectric thin film.

In the present preferred embodiment, in order to obtain excellent filter characteristics, the relationship between the relative dielectric constant ε^s₁₁/ε₀ and K²·ε^s₁₁/ε₀ and the aperture, number of pairs of electrodes, and distance between fingers is preferably in the range shown in Figs. 2 through 7.

On the surface acoustic wave substrate 22, first and second IDTs, 23 and 24, are arranged to extend along the surface acoustic wave transmission direction. The IDT 23 includes a pair of bus bars, 25 and 26. The pair of bus bars, 25 and 26, extend in the surface acoustic wave transmission direction. A plurality of electrode fingers 25a are connected to the bus bar 25, and a plurality of electrode fingers 26a are connected to the bus bar 26. The electrode fingers 25a and electrode fingers 26a are arranged so as to be interdigitated with each other. That is, the IDT 23 includes a first comb electrode made up of the bus bar 25 and the plurality of electrode fingers 25a and a second comb electrode made up of the bus bar 26 and the electrode fingers 26a.

Similarly, the IDT 24 includes a pair of bus bars, 27 and 28, extending in the surface acoustic wave transmission direction respectively, a plurality of electrode fingers 27a connected to the bus bar 27, and a plurality of electrode fingers 28a connected to the bus bar 28.

Reflectors 29 and 30 are arranged outside of the area occupied by the IDTs 23 and 24 and extend in the surface acoustic wave transmission direction. The reflectors, 29 and 30, have a construction in which both ends of a plurality of electrode fingers extending in the direction normal to the surface acoustic wave transmission direction are short-circuited.

In the resonator type surface acoustic wave filter of longitudinal type 21 of the present preferred embodiment, an input signal is applied between the bus bars, 25 and 26, of the one IDT 23, so that a BSG wave is resonated in the IDTs, 23 and 24, and an output signal based on the BGS wave confined between the reflectors, 29 and 30, is taken out from the bus bars, 27 and 28, in the IDT. In this case, because the IDTs, 23 and 24, are set to be in close vicinity to each other in the surface acoustic wave transmission direction, they function as a longitudinally coupled resonator type surface acoustic wave filter. Moreover, the distance between the IDTs, 23 and 24, that is, the distance between the center lines of the electrode finger 2Sa1 and electrode finger 28a1, the closest to each other of the IDTs, 23 and 24, becomes a coupling length x.

Because the resonator type surface acoustic wave filter 21 of the present preferred embodiment is produced so as to satisfy the above-described relationship between the relative dielectric constant ε^s₁₁/ε₀and K²·ε^s₁₁/ε₀ and the electrode aperture, number of pairs, and because the distance between fingers satisfies the conditions shown in Figs. 2 through 7, a filter having very small filter loss and excellent filter characteristics is obtained.

Accordingly, although in the longitudinally coupled resonator filter of a Rayleigh wave type using quartz, the number of pairs of electrode fingers of the IDT is preferably about 200 to about 300, in the present preferred embodiment it is understood that the number of pairs of electrode fingers may be less.

Further, by making the number of pairs of electrode fingers in the IDTs, 23 and 24, less than 110, excellent resonance characteristics can be obtained. That is explained based on the following experimental example.

Fig. 2 shows the relationship between the relative dielectric constant ε^s₁₁/ε₀of the piezoelectric material constituting the surface acoustic wave substrate 22 and the aperture Y in the IDTs, 23 and 24. The solid lines, A and B, of Fig. 2 are the lines approximated to the upper and lower limits of a set of measured points corresponding to excellent filter characteristics, as explained earlier. The solid line A of Fig. 2 is expressed by a formula of Y = 5.52 + 66.62 x exp (-E/110), and the solid line B is expressed by Y = 0.80 + 3.48 x exp (-E/404) Accordingly, by setting the aperture Y so as to satisfy the above-mentioned formula (1), excellent characteristics can be obtained. In addition, in the case of a longitudinally coupled resonator, the optimum apertures of the resonator and filter are different from each other as in the case of a transversely coupled resonator.

Both of the solid lines, A and B, in Figs. 3 through 7 are the lines obtained by approximating to the portions along the upper and lower limits of a set of measurement points corresponding to excellent filter characteristics.

Fig. 3 shows the relationship between the product K²·ε^s₁₁/ε₀ of the electromechanical coupling coefficient K² and relative dielectric constant ε^s₁₁/ε₀ of the piezoelectric material constituting the surface acoustic wave substrate 22 and the electrode finger aperture Y in the IDTs, 23 and 24. The solid line A of Fig. 3 is expressed by a formula of Y = 7.96 + 44.14 x exp (-F/38.3), and the solid line B is expressed by Y = 0.40 + 4.35 x exp (-F/80) Accordingly, as clearly understood from Fig. 3, by setting the aperture Y so as to satisfy the above formula (2), excellent filter characteristics can be obtained.

Fig. 4 shows the relationship between the relative dielectric constant ε^s₁₁/ε₀ of the piezoelectric material constituting the surface acoustic wave substrate 22 and the number of pairs of the electrode fingers in the IDTs, 23 and 24, and Fig. 5 shows the relationship between K²·ε^s₁₁/ε₀ and the number N of pairs of the electrode fingers in the IDTs, 23 and 24. The solid line B of Fig. 4 is expressed by a formula of N = 2.0, and the solid line A is expressed by N = 114.4 + 0.02 x E - 9.283 x 10^-5 x E^-2 Accordingly, by setting the number of pairs of electrode fingers so as to satisfy the above formula (3), as clearly understood from Fig. 4, excellent filter characteristics can be obtained.

Similarly, the solid line B of Fig. 5 corresponds to N = 2.0, and the solid line A is expressed by N = -171 + 292 x exp (-F/672) Accordingly, by setting the number N of pairs of electrode fingers so as to satisfy the above-mentioned formula (4), excellent filter characteristics can be obtained.

Fig. 6 shows the relationship between the relative dielectric constant ε^s₁₁/ε₀ of the piezoelectric material constituting the surface acoustic wave substrate 22 and the distance X between adjacent electrode fingers of the first and second IDTs, and Fig. 7 shows the relationship between K²·ε^s₁₁/ε₀ and the distance X between electrode fingers. The solid line A of Fig. 6 becomes X = - 5.423 + 5.994 x exp (-E/22894), and the solid line B becomes X = 0.255 + 0.19 x exp (-E/446) Therefore, by setting the distance X between electrode fingers so as to satisfy above-mentioned formula (5), excellent filter characteristics can be obtained.

Similarly, the solid line A of Fig. 7 becomes X = 0.364 + 0.198 x exp (-F/67.5), and the solid line B becomes X = 0.241 - 0.169 x exp (-F/58.1) Accordingly, by setting the distance X between electrode fingers so as to satisfy the above formula (6), excellent filter characteristics can be obtained.

From Figs. 3 through 7, it is understood that by variously changing the electrode finger aperture, distance between adjacent electrode fingers of the first and second IDTs, and number of pairs of electrode fingers in accordance with the value of the relative dielectric constant ε^s₁₁/ε₀ or K²·ε^s₁₁/ε₀ of the surface acoustic wave substrate, excellent resonance characteristics can be obtained.

It is to be noted that Fig. 1 depicts the longitudinally coupled resonator type surface acoustic wave filter having a pair of reflectors, but the present invention is also applied to an edge reflection longitudinally coupled resonator type surface acoustic wave filter.

Further, in the filter according to the first embodiment of the present invention, the IDT may be divided into a plurality of sub-IDT portions connected in series with each other.

The present invention can be suitably applied to various electronic components, apparatuses or devices utilizing a surface acoustic wave filter in which the unique features of the present invention are successfully used. For example, the present invention may be applied to a duplexer and communication apparatus including the duplexer.

Fig. 8 is a block diagram of a communication apparatus 50 having a duplexer 40. The communication apparatus 50 may be a cellular phone, for example, because a cellular phone which usually requires a small hand-held body and a high efficiency is suitable to enjoy the aforementioned merits of the filter according to various preferred embodiments of the present invention.

The communication apparatus 50 preferably includes duplexer 40, an antenna 51, a receiver 52 and a transmitter 53. The duplexer 40 preferably includes a SAW filter 41 and a SAW filter 42, and one end of each of the SAW filter 41 and the SAW filter 42 are connected in parallel to define a first terminal 43. The other ends of the SAW filter 41 and the SAW filter 42 are connected to a second terminal 44 and a third terminal 45, respectively. The SAW filter 41 and the SAW filter 42 may be any one of the SAW filters explained in the aforementioned examples. The antenna 51, the receiver 52 and the transmitter 53 are connected to the first terminal 43, the second terminal 44 and the third terminal 45 of the duplexer 40, respectively.

The pass bands of the SAW filters 41 and 42 of the duplexer 40 are selected such that the signals received through the antenna 51 pass through the SAW filter 41 and are blocked by the SAW filter 42 and that the signals to be transmitted from the transmitter 53 pass through the SAW filter 42.

While the invention has been particularly shown and described with reference to preferred embodiments thereof, it will be understood by those skilled in the art that the foregoing and other changes in form and details may be made therein without departing from the scope of the invention as defined in the accompanying claims.